This invention relates to a method for fabricating an interconnection structure used in a semiconductor device.
The interconnection structure used in the conventional semiconductor device is composed of a conductor line such as metal, polysilicon, and metal silicide, a contact for coupling this conductor line with semiconductor devices or other conductor line, and an insulator for isolating the conductor line from semiconductor devices or other conductor line and to passivate the surface. Of them, for example, the metal conductor line is generally fabricated from a film of aluminum (Al) or aluminum alloy (Al alloy). It has also been proposed to use a layered film of Al or Al alloy film with another metal, such as tungsten (W), tantalum (Ta), titanium (Ti) or other refractory metal, or an alloy of them with another substance, for example, an alloy with silicon (Si) (e.g., D.S. Gardner et al., IEEE Transaction on Electron Devices, Vol. 32, 1985, p.174).
It has also been proposed to fabricate a metal conductor line by laminating W films on the top and side surface of a metal line by performing chemical vapor deposition (CVD) in an atmosphere containing a W compound gas after processing the Al alloy film deposited on an insulated film into a metal line (e.g., H.P. Hey et al., Technical Digest of 1986 International Electron Device Meeting, p. 50).
On the other hand, the Al or Al alloy film has usually been laminated by a sputtering method. In addition, it has been proposed to use a sputtering method improved in step coverage by applying either bias potential or thermal energy or both to the substrate during at least a certain time of the deposition (e.g., K. Kamoshida et al., Technical Digest of 1986 International Electron Device Meeting, p. 70).
However, the interconnection structure using the metal conductor line fabricated from a film of Al or Al alloy involves the following problems, and there is a limit to its use in miniaturized semiconductor devices.
(1) A short circuit with other wiring occurs due to hillocks formed on the surface of the metal conductor line or metal film at the time of heat treatment performed in the fabricating process after the metal film deposition step. PA0 (2) When an Al alloy film containing Si is used at least as one material, Si nodule precipitation occurs in the metal conductor line, and effective conductor sectional area decreases in this portion. If the degree of this decrease is great, the conductor resistance increases. If the decrease is not so significant, for example, reliability deteriorates due to electromigration. PA0 (3) At the time of heat treatment applied in the manufacturing step after the metal conductor is covered with an insulating film, or at the time of storage or operation after completion, when the semiconductor device temperature reaches or exceeds 100.degree. C., a stress induced failure occurs. That is, voids are formed in the metal conductor, and the conductor is broken or the effective conductor sectional area is decreased in that portion, and reliability deteriorates due to electromigration. Furthermore, the above-mentioned Si nodule precipitation is enhanced. PA0 (4) Electromigration is caused by the current flowing in the metal conductor during operation of the semiconductor, and voids are formed in the metal conductor to cause breakage of the conductor or formation of hillocks on the surface of the conductor, thereby producing a short circuit with another conductor(s). PA0 (5) On the surface of the Al or Al alloy, an alumina layer which is chemically stable and is an electric insulator is easily formed, and therefore when an interconnection structure having two or more metal conductor layers is fabricated by using the Al or Al alloy film, it is difficult to completely remove the alumina layer at the interface of the contact part with the conductor of the next layer, and hence it is difficult to obtain an electrically favorable connection at a high yield. PA0 (6) The Al or Al alloy film deposited by an ordinary sputtering method is poor in step coverage, and the film thickness decreases in the stepped portion of the substrate surface, for example, in the contact holes. In particular, when the hole diameter is small and its depth is large, breakage occurs when forming a conductor line. Even if breakage does not occur when forming a conductor line, the sectional area of the metal conductor line decreases, and in such portion, when covered with a dielectric film, the mechanical stress applied on the metal conductor line becomes large. Therefore, said stress induced failure or electromigration may be easily generated in such area, which lowers reliability.
In an actual semiconductor device, moreover, an insulating film effective for preventing osmosis of moisture or other impurity which causes the characteristics of the semiconductor device to vary, such as a silicon nitride film, is often used as one of the materials for the passivation film or interlayer dielectric film, and since such film possesses a strong compressive strength, an intense tensile stress is applied to the metal conductor line so that the above problems in particular may appear.
Moreover, this phenomenon is a particularly serious problem in a semiconductor device having a high degree of integration due to the following reasons.
That is, in order to raise the degree of integration, it is important to planarize the surface of the interlayer dielectric film to isolate the individual conductor layers. To accomplish this, dielectric films fabricated from organic materials such as polyimide, spin-on glass (SOG), and silicon oxide deposited by chemical vapor deposition (CVD) in an atmosphere containing an organic Si compound gas such as tetraethylorthosilicate (TEOS), are often used as at least one of the materials for composing the interlayer dielectric film. However, such films have a high content of moisture or other volatile matter, and by reaction therewith the thickness of an alumina layer on the Al alloy film surface may increase, or a compound which is more difficult to be removed than alumina may be generated, which makes it difficult to obtain an electrically favorable connection between conductor lines at a high yield.
Of these problems, points (3) and (4) may be solved by using a laminated film of Al or Al alloy film with another metal film. But it will not be an effective solution for the other problems. Concerning the first problem, for example, although generation of hillocks on the surface of the metal film or metal conductor may be inhibited, generation of hillocks on the side surface is encouraged. Besides, as compared with a metal conductor line fabricated from an Al or Al alloy film having the same size, the resistance is higher, and this resistance is further increased by heat treatment in the manufacturing step after formation of the metal conductor line.
On the other hand, in the case of the abovesaid method of fabricating a metal conductor line by depositing a W film on the top and side surfaces of the line by CVD in an atmosphere containing a W compound gas after processing the Al alloy film depositing on the dielectric film into a metal line, problems (1), (2) and (4) may be solved.
However, as for problem (6), for example, it is not so effective. Hey et al. deposited an Al alloy film by an ordinary sputtering method in a contact hole with a tapered side 1.25 .mu.m in diameter, processed it into a line, deposited a W film, and proved that, in this structure, the step coverage of metal wiring at the side wall was improved, but in this case, too, the film thickness of the Al alloy at the side wall is far decreased as compared with that of the flat part, and there is a high possibility of electromigration or a lowering of the reliability to stress migration. Also, when the contact hole diameter is smaller or when the side wall is formed vertically, the possibility of occurrence of defects in the conductor line fabrication process becomes higher. In this case, breakage occurs at the side wall when fabricating an Al alloy line, and since the W film deposits only on the surface of the Al alloy line, it is impossible to repair the breakage occurring in the line even by a W film deposition.
Whether this method is effective for problem (3) has not yet been discussed. Further, Hey et al. only employed a silicon oxide film which is supposed to have a very slight stress as an interlayer dielectric film, and did not report the effect of using a dielectric film having a strong compressive stress.
Relating to problem (5), this method is exemplified by depositing a W film in a contact hole by using a silicon oxide film as an interlayer dielectric film, but nothing has been reported about the case in which a W film is not deposited in the contact hole, or when an organic dielectric film such as polyimide, SOG film, or silicon oxide film or other dielectric film fabricated by CVD in an atmosphere containing an organic Si compound gas is used at least as one of the materials for the interlayer dielectric film.
Problem (6) may possibly be solved by depositing an Al or Al alloy film by a sputtering method improved in step coverage by applying either bias potential or heat energy, or both, to the substrate at least during a certain time of the deposition process. However, when the conductor line is fabricated by using such Al or Al alloy film, the stress induced failure of the metal conductor and lowering of reliability due to electromigration are newly found problems as compared with the metal conductor made from a film deposited by the conventional sputtering method. Even if these problems were solved, it would be difficult to apply this method when forming second and subsequent metal conductor lines in the interconnection structure with two or more metal conductor layers. This is because defects may occur in the metal conductor substratum when depositing the Al or Al alloy film used for fabricating the second and subsequent Al alloy films by the sputtering method improved in step coverage to the tapered portion of the substrate surface.